Single quote iniset argument in merge_config_file

If we don't single quote the extracted argument to iniset we drop any
quotes from the source.  Add a simple test-case for this.

Partial-bug: #1374118

Change-Id: If2f47b64b11015e727a011c7e5f6e8ad378b90eb
Merge "Single quote iniset argument in merge_config_file"
Merge branch 'master' of https://github.com/openstack-dev/devstack